1. Field of the Invention
The present invention relates to an improvement on a seed crystal holder used in a crystal pulling apparatus for manufacturing a variety of crystal materials such as semiconductors, dielectrics, and magnetic materials in accordance with the Czochralski method.
2. Description of the Related Art
Conventionally, a single-crystal material such as a semiconductor silicon single crystal has been manufactured as a bar-shaped crystal in accordance with the Czochralski (CZ) method.
An example of a crystal pulling apparatus used in the CZ method is given in FIG. 3. The crystal pulling apparatus 20 shown in FIG. 3 comprises a crucible 3 disposed within a chamber 10 and adapted to contain a silicon melt 2; a crucible support shaft 4 and an unillustrated rotation mechanism for supporting and rotating the crucible 3; a seed crystal holder 6 for holding a seed crystal 5; a wire 7 for pulling the seed crystal holder 6 upward; and an unillustrated winding mechanism for rotating or winding the wire 7. The crucible 3 is surrounded by a heater 8, which is further surrounded by a heat-insulating cylinder 9.
Next, a method for growing a single crystal by use of the above-described crystal pulling apparatus 20 will be described.
First, within the crucible 3, polycrystalline silicon is heated to at least its melting point so as to be melted. Then, the wire 7 is unwound so that a tip of the seed crystal 5 is brought into contact with or dipped in the center of the melt 2. Subsequently, the crucible 3 is rotated in a proper direction, and the wire 7 is wound while being rotated so as to pull the seed crystal 5 upward. Thus, single-crystal growth starts. Through proper control of the speed of pulling and the temperature of the melt, a single crystal 1 having a substantially cylindrical shape can be obtained.
For the seed crystal holder 6 for holding the seed crystal 5 in the CZ method, there have been proposed and put into practical use various seed crystal holders having different structures, which are basically classified into two types: 1) a chuck type as shown in FIG. 2A, and 2) a key-lock type as shown in FIGS. 2B, 2C, and 2D. In the chuck-type seed crystal holder as shown in FIG. 2A, the seed crystal 5 is held by means of a plurality of claws 22 that open and close as the outer sleeve 27 rotates. In the seed crystal holder 6 shown in FIG. 2B, the seed crystal 5 is inserted into a bore formed at the center of the holder body; an insert 24 is inserted into the body from a side thereof so that the insert 24 comes into contact with a slanted cutaway surface of the seed crystal 5; and a key 25 is then inserted into the insert 24 so as to lock the insert. The seed crystal holder 6 shown in FIG. 2C is an improved type of the seed crystal holder shown in FIG. 2B. In the seed crystal holder 6 shown in FIG. 2C, the insert 24 is locked through use of a ring 26 having a tapered inner surface. That is, the ring 26 is fitted onto the tapered holder body of the seed crystal holder 6 from above in order to lock the insert. In the seed crystal holder shown in FIG. 2D, the seed crystal 5 is inserted into a bore formed at the center of the holder body of the cylindrical seed crystal holder 6; and an insert (pin) 24 is inserted from a side with respect to the seed crystal 5 so that the seed crystal 5 is engaged with the seed crystal holder 6.
In conventional seed crystal holders as described above, the surface roughness of a seed crystal is not always identical with that of the seed crystal holder, particularly that of the insert. Also, even when a seed crystal is engaged with and held by the seed crystal holder, there may be slight play between the contact surfaces of the seed crystal and the seed crystal holder. Thus, contact is established not through the entire contact surfaces (hereinafter referred to as "full-surface contact"), but through a single point (hereinafter referred to as "point contact"), so that load is concentrated at one point. In such a state, it is very difficult to safely hold and pull a single crystal being grown below a seed crystal and having a large diameter and weight. In extreme cases, cracks may be generated in a seed crystal holder at a point where load stress is concentrated, resulting in breakage of the seed crystal holder, or breakage of the seed crystal itself, thereby causing the single crystal being pulled to fall down or drop into a silicon melt contained in the crucible. Also, when the same seed crystal is used repeatedly after etching or cleaning, the risk of the seed crystal being broken increases with the number of repetitions.